Thermal demand meter



July 6, 1943- .A H. P. vAssAR 2,323,738 THERMAL DEMAND METER k y Filed May 2o, 1941 2 sheets-sheet 1 INVENToR Hervey/ 3 Vassar:

July -6, 1943.

H P. VASSAR i THERMAL DEMAND METER Filed May 20, 1941 2 sheets-sheei 2 `mvENToR Hefwey P Vassar. I

Patented July 6, 1943 THERMAL DEMAND METER Hervey P. Vassar, Bloomiield,vN. lJ., assignor to Westinghouse Electric Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May zo, 1941, serial No. 394,260 2o claims. (ci. 1v1-s4) This invention relates to measuring instruments, and it has particular relation to measuring instruments for both integrating,.and measuring the maximum demand of Velectrical, energy.

Maximum demand devices employed commercially fall into two general classes, block-interval 'and time-lagged. In the block-interval maximum demand device, the maximum demand pointer moves across its scale at a constant heat conductive paths for conducting heat to rate when the maximum demand device is subthe parts to be heated.

jected to a constant load. However, when .a The invention further contemplates an imtime-lagged maximum demand device is subproved construction for the thermal maximum jected to a constant load, the maximum demand demand device, To this end, a housing is propointer moves across lts scale with a speed which vided vwhich includes abase portion and a, cover diminishes in accordance with the time of deportion. Upon removal of the cover portion, the ilection. Generally, the rate of diminution of thermally responsive actuating elements and the sped of th pointer inla timtl-Ilxiligged maxitheir dassociateiil fstrucilre Ieadiily may fbe giemum emand ev ce s a ogari ic or expomove as a un .rom e ous ng. Pre era y, nential function. Examples of the block-iterthe base portion of the housing is formed as a val and time-lagged maximum demand devices single unit in order to reduce cost and to assure are shown respectively in the patents to Rf H. alignment of the various parts of the maximum Lewis et al., No. 2,047,376, and B. H. Smith, No. demand device.

1,417,695, both of which are assigned to the It is, therefore, an object of the invention to Westinghouse Electric & Manufacturing Comprovide an improved measuring device including pany. a thermal maximum demand device of improved 'Ihe time-lagged maximum demand measuraccuracy. ing device closely follows the heating and cool- It is a further object of the invention to proing curves of electrical apparatus. For this and vide an improved casing for a thermal maximum other reasons, such a device is often desirable. 50 Ydemand device. thPrior art maximum demand devices of the It is another object of the invention to provide ermal time-lagged type have required exa thermal maximum demand device having tremely intricate and tedious assembly. Not readily accessible parts. I, only does such construction contribute to the It is an additional object of the invention to initial COSUO the device. but it greatly CO'mDl- 35 provide a thermal maximum demand device havcates the problems encountered in servicing such ing improved ambient temperature compensadevices. Moreover, 1t has been customary to ention, close the thermal maximum demand device m a It is sun another object of the invention u) ierasing parate fgm t'llt enclosig anfintegrat- 40 provide a thermal maximum demand device g.wa our me ,r' e provls on o separate having improved means for transmitting heat casings for these 1nstrulrnentalities substantially between the heating means and the therm gggotrla requirements thereof and 'the responsive elements comprising the device.

In accordance with the invention, a thermal Otfher objects of the invention Wm be appar' maximum demand device is enclosed with a vent triom the following description,v read in con.. measuring instrument such as an integrating june fm with the accompanying drawings' in watthour meter in a common casing. Preferwhich' ably, the maximum demand device is energized Figure 1 is a' sectional View of a' measuring de in part from the electromagnet of the watthour Vice embodying the invention; meter, Since the windings employed for ener Fig. 2 is a view, in front elevation, with parts gizing the thermal maximum demand device broken away of the device illustrated in Fig. 1; generally have an appreciable temperature co- Fig- 3 is an exploded View in Perspective 0f efficient of resistance, an ambient temperature a maximum demand device embOdYing the 1nerror is introduced thereby. This error is comvention; and y pensated by forming the heatersof the thermal Fig. 4 is alschematlc View showing circuit conmaximum demand device of a material having a temperature coeilicient of resistance larger than that of the windings.

In the prior art, reliance has been placed on heat insulation for preventing undesirable heat transfer Abetween parts of the thermal maximum demand device. By following the teachings of the present invention, such undesired transfer may be minimized by providing relatively good nections for the device illustrated in Figs. 1 and 2.

Referring to the drawings, Fig. l shows a measuring instrument, such as a watthour meter I, attached to a base plate 2 by means of suitable pillars I. The watthour meter may be of generally conventional construction, including an electromagnet 4 having a voltage winding 5 and current windings 5 which cooperate when energized to produce a shifting magnetic field. An electroconductive armature or disk 1 is positioned for rotation in the field produced by the voltage and current windings. Rotation of the amature I is retarded by a braking magnet I. A conventional register 9 may be associated with the armature 'I for integrating the rotation thereof. Preferably, the register is detachably associated with the watthour meter I, a suitable construction for this purpose being shown in the Miller et aL Patent 1,598,489, which is assigned to the Westinghouse Electric I: Manufacturing Company.

'I'he casing for the watthour meter I may vary appreciably in construction. In the specific embodiment illustrated in Fig. l, the casing is designed to provide a detachable watthour meter. For this purpose, the base plate 2 is provided with contact blades III which extend through the base plate but are insulated therefrom. These contact Ablades are connected to the voltage and current windings through suitable conductors Il. The casing also includes a cover I2 which may be of glass. This cover is detachably associated with the base plate 2. It will be understood that the watthour meter I is designed to be mounted on a watthour meter socket with the contact blades III engaging contact jaws positioned within the socket (not shown). A suitable construction for the casing and the socket of a detachable watthour meter is shown in the Bradshaw et al. Patent 1,969,499, which is assigned to the Westinghouse Electric 8: Manufacturing Company.

In order to measure the maximum demand of electrical energy supplied through the watthour meter I, a maximum demand measuring device Il is associated with the watthour meter I within the same cover I2. This device may be mounted on a shelf I4 which is attached to a face plate I5 and to the watthour meter I. face plate I5 is provided with an opening I5a through which the integrating register 5 is exposed and through which the register may be moved readily for attachment, servicing or replacement.

'I'he exact construction of the maximum demand measuring device I3 may vary appreciably but in the specific embodiment illustrated in "Figs, i, 2 and 3, the device includes two bimetallic spiral springs I5 and I1 which have their inner ends attached to hubs Il and I l. These hubs are xed to a common shaft which carries a pusher arm 20a for rotation therewith. It will be understood that a bimetallic spring is formed of two dissimilar metals or alloys having different coefcients of thermal expansion.

Consequently, when each of the bimetallic springs is heated, its inner end tends to rotate relative to the outer end. The outer ends of the bimetallic springs I5 and I1 are xed in permanent positions by means which will be described below.

For controlling the temperatures of the bimetallic springs I5 andv I1, four heaters 2|, 22, 2l and 24 are associated therewith. Each of the Preferably the` bimetalllc springs is heated by one pair of heaters, as clearly illustrated in Fig. 1.

The blmetallic springs I5 and I1 are so mounted that when heated they tend to urge the shaft 2li in opposite directions oi' rotation. Conse quently, variations in temperature which affect both springs equally have no appreciable effect on the rotation of the shaft 25 and the pusher arm 20a associated with the shaft. This means that ambient temperature variations have little enect on the accuracy of the maximum demand measuring device.

Rotation of the shaft 20 and ofthe pusher arm 20a carried thereby is determined by the difference in temperatures of the bimetallic springs I5 and I1. By proper energization of the heaters, lthe rotation `of the shaft and the pusher arm may be made dependent on energy flowing through the watthour meter I. Connections suitable for this purpose are illustrated in Fig. 4.

Referring to Fig. 4, the voltage winding 5 and current winding l of the watthour meter I are shown associated with a circuit 25 for the purpose of measuring energy flowing therethrough. The heaters 2l, 22, 2l and 24 are connected in a series circuit for1energization by a current L which varies in accordance with the voltage of thecircuit 25. Although the heaters c'ould be with the current I of the circuit 25.

connected to the circuit directly or through a separate transformer, an appreciable saving in space and cost may be eected by energizing the heater from the voltage winding 5 of the watthour meter I. For this purpose, the voltage pole of the watthour meter i is provided with an auxiliarysecondary winding 2i. This auxiliary winding 2B constitutes the secondary winding of a transformer in which the voltage winding 5 of the watthour meter is the primary .winding ConsequentLv, the output of the secondary winding 25 may be represented by the current L which varies in accordance with the voltage of the circuit 25.

Each of the heaters 2|, 22, 23 and 24 also is heated by a current I1 which varies in accordance This current may be obtained by connecting one terminal of the current winding 5 to a centrally disposed tap 21 on the secondary winding 25. By inspection oi' Fig. 4, it will be noted that the heaters 2i and 22 and the heaters 2l and 24 form two arms of a parallel circuit which is connected in series lwith the current winding 6 of the watthour meter for energization by the current I flowing in the circuit 25. f Consequently, the current Ii in each heater is equal to one-half of the current I flowing in the circuit 25.

Instantaneous directions of the flow for the currents L and I1 are indicated by arrows in Fig. 4. It will be observed that the directions of flow are such that the currents Is and I1 add vectorially in the heaters 23 and 24 and subtract vectorially in the heaters 2| and 22. Consequently, when current flows in the circuit 25, a larger resultant current flows in the heaters 2l and 24 than in the heaters 2i and 22. With a circuit as illustrated in Fig. 4, the rotation of the shaft 20 and the pusher arm 20a of the maximum demand measuring device Il is dependent upon energy ilowing in the circuit 25, as well understood in the art. Other connections are shown in the aforesaid patent to B. H. Smith.

Theioperating parts of the maximum demand measuring device I3 are enclosed in a suitable housing 2l which includes a base portion 25. This base portion is provided with two chamture wherein the chambers 30 and 3| are connected by webs `39 of restricted cross section.

The restriction of the cross section is for the purpose of restricting the transmission of heat between two chambers. As a further guard against the transmission of heat between the chambers, the chambers are separated by a substantial air space 31. Moreover, it will be noted that the webs 36 are positioned as much below the bimetallic springs |6 and i1 as is practical. Such a positioning ofthe webs increases the length of the path offered to heat flowing between the two chambers and consequently serves further to reduce the heat transmission therebetween,

The base portion 29 also has a slot 38a for receiving the ring ange 38 of a bearing 39. This bearing is for the purpose of receiving one end of the shaft 20. The remaining end of the shaft is positioned in a bearing 40 carried by the face plate IE.

In order to position accurately the outer ends of the bimetallic springs |6 and I1, these ends are attached, respectively, to split rings 4| and 42. These split ringsv may be of heat conductive material such as metal or of insulating material, such as a phenol condensation product, depending upon the particular characteristic desired. In the specific embodiment illustrated in Figs. 1, 2 and 3, it may be assumed thatthe split rings 4| and 42 are of an insulating material, such as a phenol formaldehyde condensation product. The attachment of the ends to the split rings may be in any suitable manner as by rivets.

To assist inY positioning the split rings, the base portion 29 is provided with one or more ribs for each of the split rings. For example, ribs 43 are provided for the split ring 4|, and ribs 44 are provided for the split ring 42. When cach spring and its associated split ring are inserted in a chamber,the ribs 43 and 44 are received between thel ends of the split ring to position the ring within the chamber. Since the ring is somewhat resilient, the ends thereof may'be spaced apart normally by a distance slightly less than the corresponding dimension of the ribs. 'I'hls serves to eliminate play between the ribs and ring. By inspection of Fig. 1, it will be noted that the split rings 4| and 42 serve to space the bimetallic springs I6 'and |1 slightly from the wallsof the chambers provided in the housing 29.

PreferablyY an air space is left between the outer surface of each split ring and the surface of the associated chamber in order to provide increased heat insulation for the enclosed bimetallic spring. To this end, four abutments 45 may be positioned symmetrically in the base portion for each chamber to contact the associated split ring and space it from the curved wall of the chamber.

Each of the` chambers is provided with a cap 46 and 41 for completing the enclosure of each bimetallic spring. Although these caps may be associated in a unitary structure similar to the base portion 29, preferably they are completely separated in order to` increase the heat insulation between the chambers. The caps 46 and 41 asaarss bers 3l and 3| :for receiving respectively the biare provided with chambers and slots for receiving the bimetallic springs, split rings, bearing andheaters which are similar to the chambers and slots in the base portion 29. It will be noted that each of the caps includes ribs 43a and 44a which engage the outer surface of the associated split rings to complete the positioning of the rings in their respective chambers. Because of this construction, an air space is left substantiallyaround each split ring.

'I'he housing 2B maybe constructed of various /materials Preferably the material selected is portion 29 a heat and electrical insulating material,I such as a phenol formaldehyde. condensation product. Because of the accessible construction of the base portion 29 and the caps 46 andvl, these `parts maybe formed readily by-a molding or casting'operation. To facilitate such molding or casting, the various chambers and slots may taper slightly, as illustrated in Fig. 1.

With the disposition of the parts as illustrated in Figs. l and 3, heat from the various heaters is applied uniformly to all convolutions of the spiral bimetallic springs. Each of the heaters may be formed substantially as illustrated in Fig. 3. It will be noted that each heater is substantially of U-shape, having a. channel 48 communicating with a centrally disposed opening therein. Because of the aligned channel in the four heaters, the shaft 20 may be moved readily therethrough when the bimetallic spring and shaft assembly is to be Without disturbing the connections for the heaters. If desired, slits 49 may be formedA in the heaters for the purpose of increasing the resistance thereof. Moreover, each of the heaters is provided with projections 5|) which project from opposite sides of the housing 23 for the purpose of receiving the electrical connections, which connections are illustrated, for example, in Fig. 4, Y

Although the housing28 is of a heat insulating material, it is a better conductor of heat thanis air. In order to provide the best heat transmission from each heater to the associated bimetallic spring, each of the heaters preferably is urged into intimate contact with that wall of its slot which is nearest to the associated bimetallic spring. For this purpose, each of the heaters may be provided with protuberances 5| for urging vthe heater against the desired surface. Conveniently, the protuberances 5i may be formed by deforming portions of the heaters. With such a. construction, heat developed by each heater flows directly through the thin partition between the heater and its associated bimetallic spring and then across a small air space to the bimetallic spring,

Furthermore, it will be noted that the provision of the protuberances 5| provides an air space between each heater and the outer wall of the associated base portion and cap. This air space contributes further to the insulation of the chamb'er for each bimetallic spring. Since an additional air space is provided external to the exterior surface of each split ring 4| or 42. it is evident that each bimetallic spring together with its heaters are surrounded substantially by an insulating air space.

The caps 46 and 41 may be secured to the base by suitable bolts 52 which, if desired, also may be employed for securing the base portion to the shelf 4.

From the foregoing discussion, it is believed that`the assembly of the maximum demand inserted or removed measuring device Il is apparent. The base portion 2l is nrst positioned on the shelf Il. The heaters 2l, 22, 2l and 2l then are dropped into their respective slots and are connected appropriateLv in accordance with the circuit illustrated in Fig. 4, or any other suitable circuit. 'I'he bimetaliic springs I0 and I1, together with the shaft 2l, the pusher arm 20a, and the rings Il and I2 next are dropped through the channels into the base portion 2| and the shaft 2U is inserted ln the bearing 4l. Sufficient play is avuiable for the shaft 2l to permit this insertion in the bearing 4I. Before the shai't 20 is dropped t its final position, the bearing 3i may be slipped over the remaining end of the shaft for reception in the slot 31. With the parts so positioned, the caps 4l and l1 may be applied to the base portion 2l and attached in final position by means of the bolts I2. If for any reason, the maximum demand measuring device requires servicing or replacement, the caps Il and l1 may be readily removed and the bimetallic springs Il and i1, together with the shaft Il, may be removed as a unit without further disturbing the device.

The rotation of the shaft 2l may be shown in any desired manner. For example, a maximum demand pointer Il may be mounted for rotation over the face plate Il. In the specific embodiment illustrated in Fig. l. the pointer Il is fixed to a stub shaft 5l which has one end partly inserted in the bearing ll. The stub shaft N also carries a ring flange Il which is xed to the stub shaft. This ring flange is urged toward the bearing i by means of a spring Il which may be ad- Justed to exert any desired pressure by means of a screw Il. This screw 51 engages a threaded member Il carried by the face plate Il. If desired. a friction pad, such as a felt pad Il, may be inserted between the ring flange il and the bearing Il, in order to assist in holding the pointer Il in any position to which it is actuated.

A scale l0 (see Fig. 2) may be printed pn the face plate Il for cooperation with the maximum demand pointer II. In addition, a zero stop Bl may be vprovided on the face plate Il for stopping the pointer 5I in its zero or initial position. By inspection of Fig. 1, it will be noted that the pusher arm a projects through a slot in the face plate Il for engagement with the maximum demand pointer Il. Consequently, the maximum demand pointer takes a position which corresponds to the maximum rotation of the shaft 2li and the pusher arm 20. during any desired period. At the end of this period, the maximum demand pointer DI may be reset by means of a resetting knob l2 which has a shaft 62a projecting through the cover I 2. The shaft carries a spring arm il on its interior end. A suitable nange 2b and nut 82e on the shaft prevent axial movement thereof. Rotation of the knob 62 by a meter reader carries the spring arm 6I into engagement with the maximum demand pointer for returning the pointer into engagement with the zero stop Bi. At this point, the spring arm Il slips over the maximum demand pointer to leave the maximum demand pointer free for further actuation by the pusher arm 20a. In accordance with the customary practice, the knob l2 may be provided with a seal (not shown) to prevent unauthorized operation thereof.

It will be noted that the register l is readily detached through the face plate Il. Although the maximum demand pointer 58 may overlie the register, the pointer may be readily displaced to one side in order to permit detachment or insertion of the register l.

In operation, the voltage winding B and the current winding l emit heat. By mounting the maximum demand device il asymmetrically relative to these windings, it is possible to vary the characteristics thereof. For exam'ple, the bimetallic spring I'l is heated in accordance with the vector sum of the currents L and Ii. When heated this spring tends to urge the shaft 2l in a caunterclockwise direction, as viewed in Pig. 2. The blmetallic spring il is heated in accordance with the vector difference between the same currents and. when heated. tends to urge the shaft 20 in a clockwise direction, as viewed in Fil. 2. Although the chambers containing the bimetallic springs are well insulated from each other, come heat is transmitted therebetween and the amount of heat transmitted therebetween varies in accordance with the temperature difference between the two chambers. This temperature difference, in turn, is dependent upon the current flowing in the associated circuit. The heat transmission between the chambers may be taken into account when calibrating the maximum demand device. However, if the bimetallic spring I1 is mounted in theA path of heat generated by the windings 5 and i, theheat from these windings tends to compensate for the heat transmitted between the two chambers thereby contributing to a more uniform Scale.

As previously pointed out, the fact that the bimetallic springs act in opposition to each other tends to make these springs self-compensating for ambient temperature values. However, ambient temperature affects other parts oi the measuring device to introduce possible sources of error. For example, the windings B and 2l may vary appreciably in resistance in response to variations in ambient temperatures. Such variations would introduce an error in the maximum demand measuring devicereadings. The variation in resistance of the windings 5 and 26 may be compensated by proper selection of the materials employed for the heaters 2i, 22, 23 and 24.

In the prior art, it has been customary to make heaters for thermal demand units of a material having a low temperature coefficient of resistance, such a Manganin, This was for the purpose of avoiding errors introduced by variations in the resistance of the heaters as the result of temperature changes.

In order to compensate for variations in resistance of the windings 5 and 28, the heaters 2I22, 23 and 24 may be formed of a material having a high positive temperature coemcient of resistance. The coemcient of the material employed for the heaters is appreciably higher than that of the material (usually cooper) employed for the windings 5 and 2B. As illustrative of a suitable material, soft iron may be employed for the heaters. Under the influence of ambient temperature changes, the resistance of the windings 5 and 28 may change but the resistance ofthe heaters 2i, 22, 23 and 24 changes by an amount suiilciently great to compensate for the variations in the windings 5 and 2E.

ting -of the maximum demand the spring $1 is attached to a screw B8 which extends through an opening provided in a post 69. 'I'he screw i8 may be moved relative tothe post 69 by rotation of a thumb nutv 10. Attachment of the post 69 to the maximum demand l measuring device may be effected in any suitable manner as by a pin 1| which extends through a pillar 12 formed on the base portion 29.

To adjust the maximum demand measuring device, the set screw 65 is loosened to permit3 the lever il to rotate relative to the shaft 20..

With the yparts in this condition, the maximum demand pointer and the pusher arm 20a are moved to their aero positions whereupon the set screw is actuated into rm engagement with the shaft'20. As a result of this step, the

. lever aligns itself with the coil spring ,B1 when justment is described more fully in the aforesaid Smith patent. Y

Since the invention is susceptible to numerous modiiications, the-invention is to be restricted only by the appended claims. y

Certain subject matter herein disclosed is disclosed and claimed in the copend/ing B. H. Smith application, Serial No. 393,343, led May 14, 1941, and assigned to the same assignee.

I claim as my invention:

l. In a. measuring device, a pair of thermoresponsive units, means diierentially responsive to the outputs of said thermoresponsive units, means for heating each of said thermoresponsive units in accordanceI with functions of a quantity to be measured, an insulating housing providing a substantially separate enclosed space for each of said thermoresponsive units and its associated heating means, and means for spacing said thermoresponsive units and said heating means from said housing to provide a separate air space within said housing substantially surrounding each of said thermoresponsive units and its associated heating means.

2. In a'measuring device, a 'pair of thermoresponsive units, means diierentially responsive to the outputs of said thermoresponsive units, means for heating each of said thermoresponsive units in accordance with functions of a quantity to be measured, an insulating housing providing a substantially separate enclosed space for each of said thermoresponsive units and its associated heating means, means for spacing said thermoyresponsive units and said heating means from said -housing to provide a separate air space within said housing substantially surrounding each of said thermoresponsive units and its associated heating means, and electrical insulating means extending between eachof said thermoresponsive units and its associated heating means for providinga relatively good heat conductive path therebetween.

3. In a measuring device, thermoresponsive means, means Vfor heating said thermoresponsive means, and housing means of insulating material -having separate chambers for said thermohousing means.

4. In a measuring device, thermoresponsive `means including a shaft. a plurality of electrical heaters for said thermoresponsive means, each of said heaters substantially surrounding said shaft but having a channel permitting insertionrand withdrawal of said shaft therethrough, and

means mounting said heaters with their channels' in alignment whereby said shaft may be withdrawn or inserted readily relative to all of said heaters.

5. In a measuring device, thermoresponsive means including a shaft, a plurality of electrical heaters for said thermoresponsive means, each of said heaters substantially surrounding said shaft but having a channel permitting insertion and withdrawal 0i said shaft therethrough, and a housing including means mounting said heaters with their channels in alignment whereby said shaft may be withdrawn or inserted readily relative to all of said heaters, said housing comprising a plurality of sections separable in a direction substantially transverse to said shaft for providing access to said thermoresponsive means.

6. In a measuring device, thermoresponsive means including a shaft positioned for rotation in accordance with variation in temperature of said thermoresponsive means, electrical heating means for said thermoresponsive means, and housing means for said thermoresponsive means and said electrical heating means, said housing means including a unitary section of electrical insulating material having separate chambers for said thermoresponsive means and for said heating means, said insulating material having a thermal conductivity higher than that of air, and said section having an inlet to the chamber for said thermoresponsive means positioned to permit mounting and removal of said thermoresponsive means relative to said section in adirection transverse to the axis of said shaft.

7. In a measuring device, a plurality of thermoresponsive means, means differentially responsive to said thermoresponsive means, hous- Aing means including a unitary section having separate chambers for respectively receiving said thermoresponsive means, and heating means for at least one of said thermoresponsive means,

the portion of said section between said chambers having a substantially reduced cross-section positioned substantially below a major portion vof said thermoresponsive means when said thermoresponsive means are disposed generally at the same horizontal level.

8. In` an electrical energy measuring device, a plurality of thermoresponsive means, means including a shaft diierentially responsive to said thermoresponsive means, a plurality o f electrical heaters effective when energized for heating said thermoresponsive means, each of said heaters being formed substantially of sheet electrical resistance material, and housing means for said thermoresponsive means, said shaft and said electrical heaters, said housing means including a unitary section of insulating material having spaced chambers for receiving said thermoresponsive means, and having spaced slots for receiving said heaters.

9. In an electrical energy measuring device,

responsive means and said heating means, said a plurality of thermoresponsive means, means 4including a shaft diiferentially responsive to said thermoresponsive means, a plurality of electrical heaters effective when energized for heating said thermoresponsive means, each of said heaters being formed substantially of sheet electrical resistance material, and housing means for said thermoresponsive means, said shaft and said electrical heaters, said housing means including a unitary section of insulating material having spaced chambers for partially enclosing said thermoresponsive means, and having spaced slots for partially receiving said heaters, and a separate cap of insulating material cooperating with said section for completing the enclosure of each of said thermoresponsive means, said caps having chamber and slot recesses for receiving `said thermoresponsive means and the associated heaters.

10. In a measuring device, thermoresponsive means for measuring a variable quantity, said thermoresponsive means having a pair of relatively movable portions, a housing for said thermoresponsive means, and readily detachable means for accurately positioning one of saidportions relative to said housing, said readily detachabie means including cooperating interiitting male and female parts on said thermoresponsive means and said housing, said female part being proportioned to receive slidably and snugly said male part for accurately positioning one oi' said portions in a predetermined assembled position relative to said housing, said male and female parts being separable by relative movement of said thermoresponsive means and said housing only in a predetermined direction from their normal assembled positions.

11..In a measuring device, thermoresponsive means including a bimetallic element, a housing for said bimetallic element, a ring member attached to said element, and cooperating means on said ring member and said housing responsive to placement of said ring member in said housing for detachably positioning said ring member relative to said housing.

12. In a measuring device, thermoresponsive means including a spiral bimetallic element, a housing for said bimetallic element, a ring member surrounding said bimetallic element and attached thereto. and cooperating male and female parts carried by said ring member and said housing for detachably positioning said ring member relative to said housing.

13. In a measuring device. thermoresponsive means including a bimetallic spring, a split ring member surrounding said bimetallic spring and attached to an end of said bimetallic spring, a housing having a chamber for receiving said ring member and bimetallic spring. said housing having a projection disposed for reception between the ends. of said split ring member for positioning said ring member and bimetallic spring re1- ative to said housing, and means for suspending said split ring in said chamber with a substantial part of the external surface of said ring spaced from the housing.

14. In a measuring device, thermoresponsive means, electrical heating means for heating said thermoresponsive means in accordance with a variable quantity, and a copper-containing electrical circuit for energizing said heating means, said electrical heating meanshaving a temperature coelcient of resistance greater than the corresponding coefficient of copper for compensating for variations in heating of said thermoresponsive means resulting from variations in temperature oi said electrical circuit.

15. In a measuring device, a plurality of thermoresponsive means, means differentially responsive to said thermoresponsive means, electrical heating means for each of said thermoresponsive means, means including an impedance for energizing said electrical heating means, said heating means having a higher temperature coefficient of resistance than the corresponding coefiicient of said impedance for compensating for variations in resistance with respect to temperature of said energizing means.

16. In a measuring device, a watthour meter vhaving a voltage Winding, a thennal maximum demand unit having electrical heating means, and means for energizing said heating means in part from said voltage winding, said heating means having a temperature -coeiilcient of resistance which is substantially larger than the corresponding temperature coefiicient of said voltage winding for compensating for variations in resistance with respect to temperature of said voltage winding.

17. In a measuring device, an actuating assembly including a shaft, and a plurality of thermoresponsive, spiral, bimetal elements having inner ends attached to said shaft; an enclosure formed of electrical insulating material for said actuating assembly, said enclosure comprising a unitary base section having a plurality of chambers for reception of said bimetal eiements, a plurality of electroresponsive heater members carried by said base and designed to sur round substantially said shaft, said heater members having aligned channels permitting insertion and removal of said actuating assembly as a unit relative to said base section, and cover means cooperating with said base section substantially to enclose said actuating assembly; and interntting, detachable male and female parts associated with said bimetal elements and said enclosure, saidinteriltting parts being effective in response to placement of said actuating assembly in said enclosure for retaining the free ends of said bimetal elements in predetermined positions relative to said enclosure.

18. In a thermal device for measuring a variable electrical quantity, a housing comprising a base portion and a cover portion, said housing having a plurality of chambers which are exposed when said base and cover portions are separated, thermoresponsive means positioned in certain ofv said chambers, electrical heating means positioned in certain of said chambers, and means for detachably positioning said thermoresponsive means in a predetermined position in said housing for the reception of heat from said electrical heating means, said positioning means comprising intertting portions on said housing and said thermoresponsive means having configurations permitting withdrawal of .said thermoresponsive means from said housing in a predetermined direction when said cover and base portions are separated.

19. In a thermal device for measuring a variable electrical quantity; an actuating assembly comprising a shaft, a pair of thermoresponsive means for actuating said shaft, and means connecting said thermoresponsive means to said shaft at axially spaced positions; a housing for said actuating assembly comprising a base section and a cover section separable in a direction substantially transverse to said shaft, means associated with said housing for mounting said shaft for rotation therein intermediate said sections, said housing having chambers for receiving said thermoresponsive means configured to permit withdrawal of said actuating assembly as a unit in a direction substantially transverse to said shaft when said sections are separated, and electrical heating means in said housing for heating at least one of said thermoresponsive means, said electrical heating means substantially surrounding said shaft and having a channel through which said shaft moves during said withdrawal.

20. In a thermal device for measuring a variable electrical quantity; an actuating assembly comprising a shaft, a pair of thermoresponsive means for actuating-said shaft, and means connecting said thermoresponsive means to said shaft at axially spaced positions; a housing for said actuating assembly comprising a base section and a cover section separable in a direction substantially transverse to said shaft, means associated with said housing for mounting said shaft for rotation therein intermediate said sections, said housinglhaving chambers for receiving said thermoresponsive means configured to permit withdrawal of said actuating assembly 

